The human hand is an extremely complex part of the human body. It comprises several fingers articulated around the palm of the hand. Moreover, each finger has several mutually articulated phalanges. Each articulation is mobile by means of muscles. The different articulations of the hand allow objects of different forms to be grasped. The relative position of the fingers and of the palm also makes it possible to produce signs or symbols making it possible to communicate a message, for example the index finger extended to point to an object or a direction, the thumb extended to signify approval, etc. The control of the movements of the fingers of the hand is therefore of particular interest for reinforcing the humanoid nature of a robot and its interaction capabilities. One difficulty lies in the large number of actuators necessary to ensure the independent control of the movements of the fingers.
It is unrealistic to independently drive the movement of each of the phalanges of each of the fingers of the hand, as much for the gripping of an object as for the communication of a visual message. The aim in practice is to give the greatest possible number of movements with the smallest number of actuators. As is known, a mechanism is said to be underactuated when the number of drivable actuators A is less than the number of degrees of freedom N, i.e. when N>A. The degree of underactuation is then defined as the difference (N−A). An underactuated hand is for example known in which four fingers with three phalanges and one finger with two phalanges can be closed by a single actuator. By making it possible to drive fourteen degrees of freedom via fourteen pivot links by means of a single actuator, such a hand exhibits a high degree of underactuation.
Efforts have been made to improve the capacity to control a hand while retaining a high degree of underactuation of the hand. The implementation of a spreader, arranged between the actuator and the fingers so as to distribute the gripping force over each of the fingers, is for example known. The hand can thus, by means of a single actuator, grasp objects of different forms. The hand can however be driven only between an open position and a closed position, the respective position of each of the fingers in the closed position depending on the form of the object grasped. If no object is interposed between the fingers, the latter close up to a position representing a closed point.
To reinforce the humanoid nature of a robot and its interaction capabilities, it remains desirable to increase the possibilities for controlling the movements of a hand while retaining a high degree of underactuation. Obviously, the solution must be able to fit into the structural and functional environment of the robot.